# std::ranges::sort

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::random_access_iterator I, std::sentinel_for<I> S, class Comp = ranges::less, class Proj = std::identity > requires std::sortable<I, Comp, Proj> constexpr I sort( I first, S last, Comp comp = {}, Proj proj = {} ); | (1) | (since C++20)
template< ranges::random_access_range R, class Comp = ranges::less, class Proj = std::identity > requires std::sortable<ranges::iterator_t<R>, Comp, Proj> constexpr ranges::borrowed_iterator_t<R> sort( R&& r, Comp comp = {}, Proj proj = {} ); | (2) | (since C++20)

Sorts the elements in the range `[`first`,` last`)` in non-descending order. The order of equivalent elements is not guaranteed to be preserved.

A sequence is sorted with respect to a comparator comp if for any iterator `it` pointing to the sequence and any non-negative integer `n` such that `it + n` is a valid iterator pointing to an element of the sequence, std::invoke(comp, std::invoke(proj, *(it + n)), std::invoke(proj, *it)) evaluates to false.

1) Elements are compared using the given binary comparison function comp.
2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first, last | - | iterator-sentinel defining the range to sort
r | - | the range to sort
comp | - | comparison to apply to the projected elements
proj | - | projection to apply to the elements

### Return value

An iterator equal to last.

### Complexity

\(\scriptsize \mathcal{O}(N\cdot\log{(N)})\)𝓞(N·log(N)) comparisons and projections, where N = ranges::distance(first, last).

### Possible implementation

Note that typical implementations use Introsort. See also the implementation in MSVC STL and libstdc++.

    struct sort_fn
    {
        template<std::random_access_iterator I, std::sentinel_for<I> S,
                 class Comp = ranges::less, class Proj = std::identity>
        requires std::sortable<I, Comp, Proj>
        constexpr I
            operator()(I first, S last, Comp comp = {}, Proj proj = {}) const
        {
            if (first == last)
                return first;

            I last_iter = ranges::next(first, last);
            ranges::make_heap(first, last_iter, std::ref(comp), std::ref(proj));
            ranges::sort_heap(first, last_iter, std::ref(comp), std::ref(proj));

            return last_iter;
        }

        template<ranges::random_access_range R, class Comp = ranges::less,
                 class Proj = std::identity>
        requires std::sortable<ranges::iterator_t<R>, Comp, Proj>
        constexpr ranges::borrowed_iterator_t<R>
            operator()(R&& r, Comp comp = {}, Proj proj = {}) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), std::move(comp), std::move(proj));
        }
    };

    inline constexpr sort_fn sort {};

### Notes

std::sort uses std::iter_swap to swap elements, whereas `ranges::sort` instead uses ranges::iter_swap (which performs ADL for `iter_swap`, unlike std::iter_swap)

### Example

Run this code

    #include <algorithm>
    #include <array>
    #include <functional>
    #include <iomanip>
    #include <iostream>

    void print(auto comment, auto const& seq, char term = ' ')
    {
        for (std::cout << comment << '\n'; auto const& elem : seq)
            std::cout << elem << term;
        std::cout << '\n';
    }

    struct Particle
    {
        std::string name; double mass; // MeV
        template<class Os> friend
        Os& operator<<(Os& os, Particle const& p)
        {
            return os << std::left << std::setw(8) << p.name << " : " << p.mass << ' ';
        }
    };

    int main()
    {
        std::array s {5, 7, 4, 2, 8, 6, 1, 9, 0, 3};

        namespace ranges = std::ranges;

        ranges::sort(s);
        print("Sort using the default operator<", s);

        ranges::sort(s, ranges::greater());
        print("Sort using a standard library compare function object", s);

        struct
        {
            bool operator()(int a, int b) const { return a < b; }
        } customLess;
        ranges::sort(s.begin(), s.end(), customLess);
        print("Sort using a custom function object", s);

        ranges::sort(s, [](int a, int b) { return a > b; });
        print("Sort using a lambda expression", s);

        Particle particles[]
        {
            {"Electron", 0.511}, {"Muon", 105.66}, {"Tau", 1776.86},
            {"Positron", 0.511}, {"Proton", 938.27}, {"Neutron", 939.57}
        };
        ranges::sort(particles, {}, &Particle::name);
        print("\nSort by name using a projection", particles, '\n');
        ranges::sort(particles, {}, &Particle::mass);
        print("Sort by mass using a projection", particles, '\n');
    }

Output:

    Sort using the default operator<
    0 1 2 3 4 5 6 7 8 9
    Sort using a standard library compare function object
    9 8 7 6 5 4 3 2 1 0
    Sort using a custom function object
    0 1 2 3 4 5 6 7 8 9
    Sort using a lambda expression
    9 8 7 6 5 4 3 2 1 0

    Sort by name using a projection
    Electron : 0.511
    Muon     : 105.66
    Neutron  : 939.57
    Positron : 0.511
    Proton   : 938.27
    Tau      : 1776.86

    Sort by mass using a projection
    Electron : 0.511
    Positron : 0.511
    Muon     : 105.66
    Proton   : 938.27
    Neutron  : 939.57
    Tau      : 1776.86

### See also

ranges::partial_sort (C++20) | sorts the first N elements of a range (algorithm function object)
ranges::stable_sort (C++20) | sorts a range of elements while preserving order between equal elements (algorithm function object)
ranges::partition (C++20) | divides a range of elements into two groups (algorithm function object)
sort | sorts a range into ascending order (function template)

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